Warning device for radio receivers



Dec. 14, 1965 B. WEINSTEIN 3,223,927

I WARNING DEVICE FOR RADIO RECEIVERS Filed Nov. 1, 1962 1820932308 fleflwd Weiawieiau ail'owzey United States Patent 3,223,927 WARNING DEVICE FOR RADIU RECEIVERS Bernard Weinstein, 872 Chestnut St., Newton, Mass. Filed Nov. 1, 1962, Ser. No. 234,640 3 Claims. (Cl. 325401) This invention relates in general to radio receivers and pertains more particularly to devices for preventing a receiver from being accidentally left turned on when not in use.

Radios are customarily equipped with a grain or volume control rheostat operated by a knob which is turned to increase or decrease the volume of sound. Such controls frequently have the shut-oft switch for the radio incorporated in them. The switch is arranged in such a Way as to be opened when the knob is turned as far as it will go in the direction of decreasing volume, usually counter-clockwise. If the operator does not turn the knob quite far enough, the radio will be left running and, as the switch is at the low volume end of the control, the volume of sound may be too low to be noticeable. If the radio is accidentally left running for long periods of time, the life of the various components is shortened and, in the case of battery powered radios particularly, the batteries are rapidly worn out.

Radios equipped with a saparate on-off switch may also he accidentally left running in much the same manner if the operator turns the volume down to an inaudible level and forge-ts to turn off the switch.

The principal object of this invention is to provide a device which automatically emits a warning signal to attract the operators attention when the radio is left running at inaudible or barely audible volume. Another object is to provide a device which can be readily installed in most of the common types of radio sets at a minimum of expense, and which in no way impairs the performance of the radio in the normal volume range. Other objects, advantages, and novel features of the device will be apparent from the following description.

Many of the commonly used radio amplifier circuits, of both the vacuum tube and the transistor type, employ regenerative feed-back which tends to produce a low frequency oscillation referred to as motor boating, resulting in a loud staccato sound. To remove this oscillation, which is normally considered an undesirable chracteristic of renegerative amplifiers, such circuits are provided with a suppressor or de-coupling network, such as a resistor and high capacity condenser connected in the output of the first amplification stage. According to this invention, a cut-out switch is connected in the suppressor network and is opened when the volume is turned down to a level below the normal operating range, causing motor boating to occur and produce an audible signal which warns the operator that the set is still turned on. The switch is actuated by an arm on the shaft of the volume control rheostat.

In the drawings illustrating the invention:

FIGURE 1 is a schematic diagram of a typical radio receiver circuit modified according to the invention; and

FIGURE 2 is a rear view of the volume control rheostat showing the physical arrangement of the cut-out switch and its actuating arm.

FIG. 1 shows the invention as applied to a three-stage transformer coupled radio amplifier circuit utilizing three transistors 10, 11, and 12 with their output circuits coupled to the succeeding stage by transformers 13, 14, and 15, respectively. Power for the circuit is derived from a battery 16. The input signal from the antenna is applied to input junctions 17, 18, and the output junctions 19, 20 are connected to the loud speaker in the usual manner. The base bias voltage of the transistors is supplied by ice voltage dividers consisting of pairs of resistors R1, R2, R3, R4, and R5, R6, respectively. The emitter circuits are provided with limiting resistors R7, F8, and R9 bypassed by condensers C7, C8, and C9, respectively. The gain control rheostat R10 is connected in the input circuit of transistor 11. A shut-oft switch 21 is provided.

An amplifier circuit of the type just described is subject to motor boating and to suppress this normally undesirable oscillation a suppressor, or de-coupling network, enclosed by the dotted outline 22, consisting of a resistor R11 and a condensor C11, is connected in the output circuit of the first stage transistor 10. The cutout switch 23 is connected in series with condensor C11.

The volume control rheostat R10 is a rotary rheostat of a type commonly used for the purpose, operated by a shaft 24 connected to a hand knob 25, which is mounted on the outside of the radio. In FIG. 2 the switch is seen from the rear or from inside the cabinet. As thus viewed, the knob is turned counterclockwise from the position indicated by the arrow 26, which represents the extreme low volume end of the range, to increase the gain or volume. The shut-off switch 21 may be incorporated in the volume control, as is frequently done, in such a manner as to open when the knob is turned back to the position of arrow 26. Alternatively, the shut-off switch may be a separate manual switch mounted on the radio cabinet.

According to the invention an arm 27 is attached to shaft 24. Switch 23 consists of a pair of leaf spring contacts 28 and 29, normally engaged, mounted in an insulating block 30 alongside the volume control rheostat. Contact 29 projects into the path of arm 27, and is disposed to be engaged by arm 27 and bent away from contact 28, when knob 25 is turned clockwise, near the extreme low volume end of the range. Switch 23 is so placed with respect to arm 27 that the switch opens when the volume of the receiver is turned down below the normal operating level. In the case of a built-in shut-off switch, switch 23 is disposed so as to be opened before the knob 25 reaches the position to open the shut-01f switch, which corresponds to the extreme position indicated by arrow 26.

The operation of the device is as follows:

When the volume control is turned low and the receiver is turned on by closing switch 21, which may be done in case of a built-in shut-off switch by moving the volume control slightly away from its extreme low position, switch 23 is open. Condenser C11 of the suppressor network is cut out, and the circuit is in such a condition that motorboating occurs. This produces a characteristic staccato noise in the loud speaker. When the operator turns the volume up to the normal range, that is the range in which broadcast signals received by the radio are distinctly audible, switch 23 closes and the suppressor network R11, C11 becomes effective to suppress the motor-boating oscillation. As long as the volume control remains turned up to the normal opera-ting range, the radio continues to operate in the susual manner. The warning device has no effect on the quality of reception and reproduction of the broadcast signals.

When the operator wants to turn off the radio he usually turns the volume control down. If the shut-off switch is incorporated in the volume control, he necessarily turns the volume down. With this type of control, particularly, it is easy for an operator, through haste or inattention, to turn the volume control knob to a setting at which broadcast signals become inaudible, but not quite far enough to open the Shut-01f switch. When this occurs, switch 23 opens, and the radio emits a distinctly audible signal which alerts the operator to the fact that the set is not shut off. In the case of a radio with a separate shut-ofl? switch, the operator may turn down the volume control and forget to turn off the shut-off switch.

Switch 23 being opened, the radio will emit an audible warning signal.

It is understood that the invention may be applied to any type of radio or amplifier circuit which is subject to motor-boating oscillations, and includes a suppressor network for eliminating such oscillations, and an adjustable volume or gain control. The construction and arrangement of the cut-off switch may also be varied, as long as the switch operates in such a manner as to render the suppressor network inettective when the volume or gain is set below the normal operating level.

What is claimed is:

1. An amplifier circuit of the type subject to selfinduced oscillation, having a suppressor network for suppressing such oscillation, and a manually adjustable gain control rotatable to a low gain position, said circuit including a warning device comprising a switch mounted adjacent said gain control and connected in said suppressor network and adapted, when open, to render said network inefiective to suppress said oscillation, and a member mounted on said gain control and engageable with said switch to open the same when said control is in said position.

2. A device as described in claim 1, said switch comprising a pair of leaf-spring contacts normally engaged, said gain control including a rotatable shaft, and said member being mounted on said shaft and engageable with one of said contacts to separate it from the other when said control is in said position.

3. An amplifier circuit of the type subject to self-induced oscillation, having a suppressor network for suppressing such oscillation, said circuit including a manually adjustable gain control movable in a direction of decreasing gain to a shut-off position, a shut-off switch actuated by said control in said shut-off position, and a second switch to be opened by said gain control at a second position adjacent said shut-off position, said second switch being connected in said suppressor network and adapted, when open, to render said network ineffective to suppress said oscillation.

References Cited by the Examiner UNITED STATES PATENTS 2,074,014 3/1937 Chittick et al. 32540l 2,999,928 9/1961 Haydon 325400 3,038,070 6/1962 MacDonald 325364 ROBERT H. ROSE, Primary Examiner.

DAVID G. REDINBAUGH, Examiner.

E. C. MULCAHY, R. S. BELL, Assistant Examiners. 

1. AN AMPLIFIER CURCUIT OF THE TYPE SUBJECT TO SELFINDUCED OSCILLATION, HAVING A SUPPRESSOR NETWORK FOR SUPPRESSING SUCH OSCILLATION, AND A MANUALLY ADJUSTABLE GAIN CONTROL ROTATABLE TO A LOW GAIN POSITION, SAID CIRCUIT INCLUDING A WARNING DEVICE COMPRISING A SWITCH MOUNTED ADJACENT SAID GAIN CONTROL AND CONNECTED IN SAID SUPPRESSSOR NETWORK AND ADAPTED, WHEN OPEN, TO RENDER SAID NETWORK INEFFECTIVE TO SUPPRESS SAID OSCILLATION, AND A MEMBER MOUNTED ON SAID GAIN CONTROL AND ENGAGEABLE WITH SAID SWITCH TO OPEN THE SAME WHEN SAID CONTROL IS IN SAID POSITION. 